Weighing devices with strain gauges, commonly known as strain gauge weight sensors, are used, for example, in kitchen or bathroom scales. They include a metal rod or beam constrained at one end thereof, which rod is provided at certain locations along its length with weakened areas or hinges, where the metal is less thick than elsewhere. Strain gauges are fixed to this rod at a right angle with the weakened areas or hinges.
Amongst the above-mentioned sensors, there are the so-called "self supporting" or constant moment weight sensors with strain gauges of the paralellogram type, which sensors are associated with a load supporting tray and offer the particularity of giving a response totally independent of the position--i.e. of the offset--of the load on this tray.
These sensors of the paralellogram type used up to now, consist of four strain gauges, each mounted on one of four hinges of a machined test body. These four strain gauges are then connected together electrically through small leads to form a Wheatstone bridge, the terminals of which are in turn connected to a power supply and to two measuring leads. This type of sensor has the disadvantage of being long to manufacture because of the large number of wires required, and because of the complexity of the connections and of the binding of the gauges in different areas, which all results in high manufacturing costs. On the other hand however, this bridge arrangement of the gauges is self-compensating and renders the conventional sensors insensitive to the load being offset. When the load is offset, forces of the same intensity and sign are generated in the two upper hinges of the sensor. In the conventional sensor, these forces are measured on these two hinges by gauges of opposite sign (because they are juxtaposed in the Wheatstone bridge), whereby the sum of the values measured by these two gauges equals zero; the same phenomenon occurs (with two forces of opposite sign) on the lower hinges. In theory, the self-compensation should be perfect. In practice however, it was found that a small final adjustment was nearly always necessary. This is mainly due to three types of imperfections: imperfections in the machining of the test body, imperfections in the positioning of the gauges and, finally, to a lesser extent, imperfections in the homogeneity of the material from which the test body is made. These small imperfections can be corrected by slightly retouching the hinges of the sensor by machining, so as to modify the geometry of the paralellogram.
When using weight sensors in mass-produced items such as kitchen or bathroom scales, it is imperative to reduce manufacturing costs as much as possible. This is why the possibility of reducing significantly the cost of weight sensors with strain gauges was investigated. A cost reduction could be achieved by bonding side by side two gauges on two hinges (upper or lower) or on the two hinges near to a same end of the sensor. This would result in some simplification of the bonding operation, but the complexity of the wiring would remain, and the gain would only be negligible.
When other solutions were investigated, a major difficulty was encountered. Assemblies of four strain gauges already connected electrically together to form a complete Wheatstone bridge and mounted on a single support are already known. This bridge is bonded on one of the four hinges of the test body, thereby reducing significantly the bonding and wiring operations, and hence, manufacturing costs. However, this arrangement of the gauge bridge does not ensure the self-compensation which makes the conventional sensors with four separate gauges insensitive to the load being offset. The following comparison clearly shows the magnitude of the challenge :
The deviations in the reading (or error) of the sensors when the offset of load on the tray is maximum (before the final adjustment mentioned above) are as follows :
1. Case of the so-called self-compensating conventional sensor with four gauges (before the final adjustment) deviations less than 0.1 percent.
2. Case of a sensor with four hinges and a group of four sensors connected together in a Wheatsone bridge mounted on one support placed on one of the hinges: deviations of about 10 percent.
Thus, the error in the second case is at least a hundred times greater than in the first case, which is enormous and cannot be justified by cost reduction considerations.